Post Go back to editing

ADXL1002 Accuracy Limitation

Hi engineer team,

 

I was looking on the ADXL1002 datasheet and I was wondering about something : is that significant and accurate to measure a voltage difference which is less than 2mV or 1mV with the ADXL1002 ? We know that the sensitivity is 40mV/g, and the ADXL1002 is made for a huge range +/-50g, so basically the main purpose is not to measure low variation.

I saw as well that the sensitivity due to the temperature is 5% of 40mV/g. So I assume than measuring an acceleration which is lower than 5% of 1g (which means 2mV) is not really accurate.

I am currently asking this cause we are reflecting about which ADC we are gonna use, and I am wondering if a 12bit ADC with a 4V VREF is enough (which allow us to have a resolution of 0.97mV).


Thanks for your help,

Florent

Parents
  • You are welcome!  Here is a breakdown of your questions, as I understand them.

    QUESTION>> The limits of the capability of the ADXL1002 as a vibration sensor impact the choice of the ADC because if the accuracy limit is 2mV, then it is pointless to select an ADC which got a resolution of ,less than 2mV (aka AD4000) and a 12 bit ADC is enough.

    ANSWER>>  While I appreciate your kind words, you seem to be re-stating the same position that you started with in this question, but also acknowledging our motivation for encouraging the use of a 16-bit ADC.  I am confused, but let's presume that you are still wondering if a 12-bit ADC will preserve all available performance in the ADXL1002... From a fundamental signal processing point of view, resolution is not the same thing as absolute accuracy.  In fact, they would seem to be somewhat unrelated.  In simple terms, the absolute accuracy quantifies how well the sensor will represents a vibration signal, which is much larger than the noise in its output.  Resolution represents the smallest vibration signal, which the sensor will respond to.  In the following article, we proposed that if the vibration magnitude was the same as the sensor's inherent noise, it would raise the root-mean-square of the sensor output by 3dB or 40%.   

    MEMS Vibration Monitoring: From Acceleration to Velocity | Analog Devices 

    So, to be very clear, we believe that a system will need at least 16-bits of resolution in its ADC, in order to preserve the resolution that the ADXL1002 is capable of, in a vibration monitoring application. 

    QUESTION>> The ADXL1002 is a 11khz bandwidth accelerometer, so i guess you selected 5Khz as it is an average value, am I  right ? 

    ANSWER>> No, I didn't put that much thought into selecting 5kHz for this example; I took it from the section of the datasheet, which offers the AD4000 as a complementary ADC. In a real application, this will be set by the vibration profile that the system is trying to monitor. 

    QUESTION>> Ok I got your calculations,exepted this one :  Quantization noise = q/sqrt(12). Where does the 12 come from ? 

    ANSWER>> This is a fundamental relationship in quantization theory.  Deriving this industry-recognized formula is outside of what I can support in this forum, but I can help you get started, if you want dig into its origin and derivation. Check page 57 of this reference, which lists this formula, along with a reference to one of the original books on sampled data theory, which was published in 1948.  I hope that this helps. 

    http://www.analog.com/media/en/training-seminars/design-handbooks/high-speed-design-seminar/Section1.pdf#Page=57 

    QUESTION(S)>> Your thinking make sens to me now, and I understand that the ADC noise has to be smaller than the sensor noise. So according to your calculation, the total noise of the ADXL1002 is 0.0886mV. So does it means there will always gonna be this amount of voltage which is garbage ? So this is pretty much the limit of the ADC so ? I mean that we cannot measure a voltage of less than 0.0886mV otherwise this won't make sense ? 

    ANSWER>> I am not sure that I would go as far as calling this information "garbage," but I guess that is a fair statement, in this case.  I would prefer to say that the total noise is a key factor in the resolution of the sensor. I hate to be repetitive, but I would recommend reading this article and depending on your time/depth of motivation, perhaps start digging into electronic noise theory.  Motchenbacher's "Low Noise in Electronic System Design" is a classic reference on this topic.

    MEMS Vibration Monitoring: From Acceleration to Velocity | Analog Devices 

    I hope that this helps!

Reply
  • You are welcome!  Here is a breakdown of your questions, as I understand them.

    QUESTION>> The limits of the capability of the ADXL1002 as a vibration sensor impact the choice of the ADC because if the accuracy limit is 2mV, then it is pointless to select an ADC which got a resolution of ,less than 2mV (aka AD4000) and a 12 bit ADC is enough.

    ANSWER>>  While I appreciate your kind words, you seem to be re-stating the same position that you started with in this question, but also acknowledging our motivation for encouraging the use of a 16-bit ADC.  I am confused, but let's presume that you are still wondering if a 12-bit ADC will preserve all available performance in the ADXL1002... From a fundamental signal processing point of view, resolution is not the same thing as absolute accuracy.  In fact, they would seem to be somewhat unrelated.  In simple terms, the absolute accuracy quantifies how well the sensor will represents a vibration signal, which is much larger than the noise in its output.  Resolution represents the smallest vibration signal, which the sensor will respond to.  In the following article, we proposed that if the vibration magnitude was the same as the sensor's inherent noise, it would raise the root-mean-square of the sensor output by 3dB or 40%.   

    MEMS Vibration Monitoring: From Acceleration to Velocity | Analog Devices 

    So, to be very clear, we believe that a system will need at least 16-bits of resolution in its ADC, in order to preserve the resolution that the ADXL1002 is capable of, in a vibration monitoring application. 

    QUESTION>> The ADXL1002 is a 11khz bandwidth accelerometer, so i guess you selected 5Khz as it is an average value, am I  right ? 

    ANSWER>> No, I didn't put that much thought into selecting 5kHz for this example; I took it from the section of the datasheet, which offers the AD4000 as a complementary ADC. In a real application, this will be set by the vibration profile that the system is trying to monitor. 

    QUESTION>> Ok I got your calculations,exepted this one :  Quantization noise = q/sqrt(12). Where does the 12 come from ? 

    ANSWER>> This is a fundamental relationship in quantization theory.  Deriving this industry-recognized formula is outside of what I can support in this forum, but I can help you get started, if you want dig into its origin and derivation. Check page 57 of this reference, which lists this formula, along with a reference to one of the original books on sampled data theory, which was published in 1948.  I hope that this helps. 

    http://www.analog.com/media/en/training-seminars/design-handbooks/high-speed-design-seminar/Section1.pdf#Page=57 

    QUESTION(S)>> Your thinking make sens to me now, and I understand that the ADC noise has to be smaller than the sensor noise. So according to your calculation, the total noise of the ADXL1002 is 0.0886mV. So does it means there will always gonna be this amount of voltage which is garbage ? So this is pretty much the limit of the ADC so ? I mean that we cannot measure a voltage of less than 0.0886mV otherwise this won't make sense ? 

    ANSWER>> I am not sure that I would go as far as calling this information "garbage," but I guess that is a fair statement, in this case.  I would prefer to say that the total noise is a key factor in the resolution of the sensor. I hate to be repetitive, but I would recommend reading this article and depending on your time/depth of motivation, perhaps start digging into electronic noise theory.  Motchenbacher's "Low Noise in Electronic System Design" is a classic reference on this topic.

    MEMS Vibration Monitoring: From Acceleration to Velocity | Analog Devices 

    I hope that this helps!

Children
No Data